scripts/spelling.txt: add "an union" pattern and fix typo instances

Fix typos and add the following to the scripts/spelling.txt:

  an union||a union

Link: http://lkml.kernel.org/r/1481573103-11329-5-git-send-email-yamada.masahiro@socionext.com
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/tools/perf/util/sort.h b/tools/perf/util/sort.h
index 7aff317..796c847 100644
--- a/tools/perf/util/sort.h
+++ b/tools/perf/util/sort.h
@@ -108,7 +108,7 @@
 		/*
 		 * Since perf diff only supports the stdio output, TUI
 		 * fields are only accessed from perf report (or perf
-		 * top).  So make it an union to reduce memory usage.
+		 * top).  So make it a union to reduce memory usage.
 		 */
 		struct hist_entry_diff	diff;
 		struct /* for TUI */ {